High lag, high sensitivity target having solid antimony oxysulphide and porous antimony trisulphide layers



April 18, 1967 J. F. HEAGY 3,315,108 HIGH LAG HIGH SENSITIVITY TARGET HAVING SOLID OXYSULPHIDE AND POROUS ANTIMONY TRISULPHIDE I Filed Dec. 17, 1963 ANTIMONY JAYERS INVENTOR. JOHN E HEAGY A TTOR NEY United States Patent 3,315,108 HIGH LAG, HIGH SENSITIVITY TARGET HAV- ING SOLID ANTIMONY OXYSULPHIDE AND POROUS ANTIMONY TRISULPHIDE LAYERS John F. Heagy, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Dec. 17, 1963, Ser. No. 331,226 2 Claims. (Cl. 313-65) This invention relates to photoconductive pickup tubes of the vidicon type, and in particular concerns the target of such tubes.

The vidicon type tube normally includes an evacuated envelope having an electron gun in one end thereof adapted to produce a beam of electrons. In the other end of the envelope is disposed a target comprising a conducting signal plate on which is deposited a layer of photoconductive material. The beam is caused to scan the target, for example, by a system of electromagnetic deflecting coils positioned outside of the tube envelope. During operation of a vidicon tube of the type under consideration, the signal plate upon which the photoconductive layer is deposited is operated at a positive potential, which may, for example, be 30 volts. When an image is focused upon the photoconductive layer, the lighter portions of the image render elemental areas of the ph0- toconductive layer conductive. Such conductivity causes the potential of the surface of the gun side of the elemental areas aifected, to rise to the positive potential of the signal plate. The remaining elemental areas of the photoconductive layer remain at a potential that is negative with respect to the signal plate.

When the gun side of the photoconductive layer with its positive potential pattern is scanned by the electron beam, electrons from the beam are deposited on the positive areas of the photoconductive layer until the surface potential on the gun side is reduced to that of the cathode. Further electrons are turned back to form a return beam which is not utilized. Deposition of electrons on the scanned surface of any particular element of the layer causes a change in the difference of potential between the two surfaces of the element. The two surfaces of the element constitute in effect a charged capacitor. Therefore, when the two surfaces are connected through an external signal plate circuit and the scanning beam, at current is produced which constitutes the video signal.

Correspondence between the video signal and the initial image results only if the charge pattern produced by the image on the photoconductor is unimpaired until the video signal is produced, Such impairment may result, for example, from secondary emission at a ratio greater than unity from the photoconductive layer in response to impingement thereon of electrons from the electron beam. Ifa greater than unity ratio persists over the entire screen,

.all elemental areas have a positive potential with respect to the cathode. This produces one of two results. Either an appreciably reduced capacitive current is produced at elemental areas of the photoconductor having a potential between that of the cathode and the signal plate, or no capacitive current at all results if all elemental areas are at the potential of the signal plate. This latter condition is known as flip-over.

In some applications the rate of decay of a signal after light has been removed, is important. This rate of decay 3,315,108 Patented Apr. 18, 1967 is known as lag. Where the rate of decay is relatively slow the lag is high, and where the rate of decay is fast, the lag is low.

Two conditions of the photoconductor affect lag. One is the nature of the material of the photoconductor. Thus some photoconductive materials are characterized by relatively rapid changes in photoconductivity in response to light. Such photoconductor-s have low lag. Other photoconductive materials respond more slowly to light and are known as high lag materials.

Another type of lag is related to the physical form of a photoconductive material. This is known as capacitive lag. This type of lag depends upon the porosity of the photoconductive layer. A solid layer has a higher capacitive lag than a porous one.

In each case a high lag (or a slow rate of decay of the signal after removal of light), results from the inability of the beam to return the scanned surface of the photoconductor to cathode potential in one or more scans, For some types of use of a vidicon tube it is desirable that the photoconductive material of the target be characterized by relatively high sensitivity long-time lag, and a secondary emission ratio less than unity.

It is therefore an object of this invention to provide a novel target for a vidicon that is characterized by relatively high sensitivity and lag with a relatively low secondary emission ratio.

Applicant has found that solid antimony oxysulfide has the desired high sensitivity and long-time lag. However, this material is characterized by a secondary emission ratio that is appreciably greater than unity. This high secondary emission ratio adversely affects the electrostatic charge pattern on the target. At relatively low target voltage, the high secondary emission ratio of the target material causes the target to flip-over.

To solve this problem, applicant provided a transparent film of conducting material on the inner surface of an insulating substrate, such as the faceplate of the tube, a layer of solid antimony oxysulfide over the conducting film, and a relatively thin layer of porous antimony trisulfide over the solid antimony oxysulfide layer. This structure provides a target wherein the secondary emission ratio is drastically reduced without significantly changing the high sensitivity and long-time lag values.

The invention will be more clearly understood by reference to the accompanying single sheet of drawing wherein:

FIG. 1 is a sectional view of a pickup tube made in accordance with this invention; and

FIG. 2 is an enlarged sectional view of a portion of the target employed in the tube shown in FIG. 1.

The photoconductive type pickup tube or vidicon 10 shown in FIG. 1 is an example of a tube embodying the present invention. The tube 10 comprises an evacuated envelope 12 which may be made of glass, having an electron gun assembly 14 comprising the usual cathode, grid and accelerating electrode positioned in one end for producing an electron beam, and at the other end an accelerating electrode 16. By means of potentials applied to the electron gun 14 and to the accelerating electrode 16, as well as by means of appropriate magnetic fields from conventional alignment coils, focus coils and defleeting yoke (none of which are shown for simplicity of illustration) the electron beam from electron gun assembly 14 is directed toward and scanned over a photoconductive target electrode 18, positioned at the said other end of the tube envelope on the side of the accelerating electrode 16 remote from the gun. Other known electrostatic deflection and focusing means can be used instead of the magnetic system referred to.

The target electrode 18 is disposed on the inner surface of a faceplate 22 which forms an end of the envelope 12. The faceplate 22 is sealed across an end of the envelope 12 by means of a sealing ring 24 which may be made of indium. The conductive sealing ring 24 serves an additional function in connection with the target to be described.

The target 18, as shown in more detail in FIG. 2 comprises a film 20 of conducting material deposited on the inner surface of faceplate 22. This material has a sufficient lateral extent to contact the sealing ring 24. The conductive film or electrode 20 is made of a material that is selected for its transparency to radiations of particular Wavelengths of interest, and for its electrical conductivity. For the visible range of wavelengths, a layer of tin oxide is suitable. This layer of conductive material may have a thickness of about 1 micron. The fact that the transparent conductive coating 20 is deposited in contact with the electrically conductive sealing ring 24 causes the sealing ring to serve as an output electrode for potentials applied to the transparent conductive electrode 20. Accordingly, the transparent conductive electrode 20 may function as a signal plate for obtaining output signals from the device 10.

On the transparent electrically conductive electrode 20 there is deposited a layer of photoconductive material 26. This layer of photoconductive material consists of solid antimony oxysulfide. The layer of solid antimony oxysulfide is deposited on the transparent conductive electrode 20 in an ambient having a pressure of approximately X mm. mercury.

One way in which the antimony oxysulfide may be applied to the transparent conductive electrode is to cause a face-plate having the conductive electrode thereon to travel past a boat containing antimony oxysulfide at a linear speed of 0.41 inch per second with the boat at a temperature of approximately 550 C. The path of the faceplate is about 2 inches from the boat. The speed with which the faceplate travels past the boat is a function of the thickness of the coating of antimony oxysulfide to be applied, and it has been determined that at the speed indicated the thickness of the antimony oxysulfide layer is about 2.5 microns. The relatively low pressure of the atmosphere within which the antimony oxysulfide is deposited, results in a solid layer of this material.

The foregoing thickness of the layer 26 of antimony oxysulfide was determined by the metallurgical microscope technique. Other thickness values of antimony oxysulfide layers were determined by a ratio computation. This computation recognizes that the ratio between the different speeds of travel of the faceplate is equal to the ratio between the thicknesses of the layers applied at the two speeds. Thus, when one speed and the thickness of the layer formed as a consequence thereof, are known, it is possible to compute the thickness of another layer from the speed at which the faceplate traveled during the application of such another layer. This ratio computation requires that the temperature of the boat and its distance from the path of travel of the faceplate remain unchanged.

Measurements made in this manner of the thicknesses of various antimony oxysulfide layers reveal that for satisfactory results, the thickness of the layer should be within the range of from about 1 to about 4 microns, and a preferred value is 2.5 microns. This preferred thickness value of about 2.5 microns is optimum in that it is the best compromise between sensitivity and peeling of the layer of antimony oxysulfide. A reduction in thickness below this value is accompanied by a reduction in ondary emission characteristic of this layer.

sensitivity. Thickness less than about 1 micron has such poor sensitivity as to be unacceptable. A thickness above this preferred value tends to cause peeling of the layer. This tendency toward peeling is sufficiently low so as to be acceptable up to a thickness of about 4 microns of the antimony oxysulfide layer. Any further increase in thickness above 4 microns renders coating peeling prohibitive.

A relatively thin layer 27 of antimony trisulfide is then deposited over the antimony oxysulfide layer 26. The application of this layer of antimony trisulfide is effected in an atmosphere of argon in a pressure of about 275 microns of mercury. In one example, the layer of antimony trisulfide was applied by providing a boat containing the antimony trisulfide and maintained at a temperature of approximately 450 C. The faceplate having thereon the conducting layer 20 and the layer 26 thereover of solid antimony oxysulfide, was caused to travel past the boat on a track spaced 2 inches from the path of travel of the faceplate, at a track speed of 1.67 inches per second. The resultant layer of antimony trisulfide is so thin that it cannot be measured accurately by optical methods. Its thickness however has been determined by other methods to be about 0.3 micron.

This thickness was determined by a ratio computation, as described in the foregoing, in which the ratio between one speed of the faceplate producing a measurable thickness, and another and higher speed producing a layer thickness that can be determined only by a ratio computation. Such ratio computation and tests with the resulting screens revealed that the thickness of the antimony trisulfide layer should be restricted to a range of from about 0.1 to about 0.6 micron, for best results, and preferably should be about 0.25 micron.

This preference is dictated by the fact that the antimony trisulfide layer is relied on to produce a surface characteristic only that has a reduced ratio of secondary emission without affecting the sensitivity of the antimony oxysulfide layer. Any increase in the thickness of the antimony trisulfide layer above about 0.25 micron has an adverse affect on the sensitivity of the antimony oxy sulfide layer. This adverse effect can be tolerated up to a thickness of about 0.6 micron. Any decrease in thickness of the antimony trisulfide layer below the preferred thickness of about 0.25 micron, adversely affects the sec- A thickness less than about 0.1 micron cannot be tolerated because it is accompanied by flip-over.

Since the layer 27 of antimony trisulfide is deposited in an ambient of greater gas pressure than that in which the antimony oxysulfide was deposited, the antimony trisulfide layer is appreciably more porous than the antimony oxysulfide.

Applicant has found that the relatively thin layer of porous antimony trisulfide does not significantly change the sensitivity and time lag values of the solid antimony oxysulfide layer. However, it does appreciably reduce the secondary emission ratio to a value less than unity.

Accordingly, there is provided a pick-up tube target of relatively high sensitivity and long-time lag coupled with a relatively low characteristic of secondary emission.

What is claimed is:

1. A pickup tube having:

(a) a glass faceplate,

(b) a high lag, high sensitivity target on the inner surface of said faceplate, said target consisting'of:

(1) a first relatively thin transparent layer of metal,

(2) a second layer of solid antimony oxysulfide having a thickness of from 1 to 4 microns, on said first layer, and

(3) a third layer of porous antimony trisulfide having a thickness of from about 0.1 to about 0.6 micron on said second layer.

2. A pickup tube having:

(a) an insulating faceplate,

(b) a high lag, high sensitivity target on the inner surface of said faceplate, said target consisting of:

(1) a transparent first layer of conductive material,

(2) a second layer of solid antimony oxysulfide having a thickness of about 2.5 microns, on said first layer, and

(3) a third layer of porous antimony trisulfide References Cited by the Examiner UNITED STATES PATENTS Forgue 31365 X Forgue et a1 313-65 Cope 31394 X Lubszynski et a1. 313-65 X Lubszynski et al. 313-101 X Lubszynski 3l3-65 X having a thickness of about 0.25 micron, on said 10 JAMES W. LAWRENCE, Primary Examiner.

ROBERT SEGAL, Examiner.

second layer. 

1. A PICKUP TUBE HAVING: (A) A GLASS FACEPLATE, (B) A HIGH LAG, HIGH SENSITIVITY TARGET ON THE INNER SURFACE OF SAID FACEPLATE, SAID TARGET CONSISTING OF: (1) A FIRST RELATIVELY THIN TRANSPARENT LAYER OF METAL, 